The safety of this system stems from its
low voltage level, which should never exceed 50 V ac or
120 V dc, and is too low to cause enough current to flow
to provide a lethal electric shock. The reason for the difference
between ac and dc levels is shown in {Figs
3.9
& 3.10}.

It is not intended that people should make
contact with conductors at this voltage; where live parts
are not insulated or otherwise protected, they must be fed
at the lower voltage level of 25 V ac or 60 V ripple-free
dc although insulation may sometimes he necessary, for example
to prevent short-circuits on high power batteries. To qualify
as a separated extra-low voltage (SELV) system, an installation
must comply with conditions which include:

1. - it must be impossible for the
extra-low voltage source to come into contact with a low
voltage system. It can be obtained from a safety isolating
transformer, a suitable motor generator set, a battery,
or an electronic power supply unit which is protected against
the appearance of low voltage at its terminals,

2. - there must be no connection
whatever between the live parts of the SELV system and earth
or the protective system of low voltage circuits. The danger
here is that the earthed metalwork of another system may
rise to a high potential under fault conditions and be imported
into the SELV system.

3. - there must be physical separation
from the conductors of other systems, the segregation being
the same as that required for circuits of different types
{6.6},

4. - plugs and sockets must not
be interchangeable with those of other systems; this requirement
will prevent a SELV device being accidentally connected
to a low voltage system,

5. - plugs and sockets must not
have a protective connection (earth pin). This will prevent
the mixing of SELV and FELV devices. Where the Electricity
at Work Regulations 1989 apply, sockets must have an earth
connection, so in this case appliances must be double insulated
to class II so that they are fed by a two-core connection
and no earth is required,

6. - luminaire support couplers
with earthing provision must not be used.